Biological treatment of wastewater or sewage is known and has been used for some time to remove solids and clarify the wastewater for reuse or for safer disposal. A number of different system configurations are possible. In one configuration, the wastewater is in a tank, and at the top or bottom of the tank is a mechanism for dispersing a fluid into the wastewater. The term “fluid” as used herein, is intended to include a substance, such as a liquid or gas, that is capable of flowing and that changes its shape at a steady rate when acted upon by a force tending to change its shape. The dispersed fluid used in wastewater treatment is most commonly air.
The fluid dispersion mechanism at or near the bottom of the tank typically effects an upward movement of the wastewater in the tank. This movement or rolling of the wastewater is important to keeping the suspended solids in suspension and is essential to effect the needed mixing of the tank contents for the biological process which is carried out in the tank. The use of a dispersed fluid, such as air, is intended to supply the microorganisms of the biological process with the required dissolved oxygen. For that purpose, the air is diffused or discharged as bubbles of a predetermined size to provide the maximum possible air-water interface area per volume of air. The partial pressures of the free and dissolved oxygen then determine the rate of transfer of oxygen from the air to the water. The length of time the bubbles remain in the liquid is in part a function of the efficiency of the tank.
Known systems include a number of different configurations for dispersing a fluid into wastewater. Some configurations include a mechanism positioned within the wastewater (e.g., submerged), and other configurations include a mechanism positioned on top of the wastewater (e.g., rotating contactor). These systems are generally complicated assemblies that require extensive time and effort to manufacture, transport, and install.
FIG. 1 shows a side perspective view of a partially cutaway fine bubble diffuser assembly 100 that is conventionally used in modern wastewater treatment facilities for “submerged” treatment of the wastewater. Wastewater treatment with such assemblies is described in, as just one example, F. L. Burton, Wastewater Engineering (McGraw-Hill College, 2002), which is hereby incorporated by reference herein. When in use, a plurality of diffuser assemblies is arrayed on several lateral distribution conduits that cross a wastewater treatment tank. Diffuser assemblies may, for example, be placed every foot along a given lateral distribution conduit. A blower located near the tank sends compressed air to the lateral distribution conduits via several support distribution conduits (e.g., drop distribution conduits and manifold distribution conduits). In the diffuser assembly 100, a flexible diffuser membrane 110 sits atop a diffuser body 120. In this case, the flexible diffuser membrane 110 comprises a disc-shaped membrane that is constructed of rubber or other similar materials, which is punctured to provide a number of perforations in the form of holes or slits. The diffuser body 120 itself comprises a threaded connector 130, an air inlet orifice 140, and a receiving surface 150 for coupling to a retainer ring 160. The retainer ring 160 holds the flexible diffuser membrane 110 against the diffuser body 120. When gas is applied to the flexible diffuser membrane 110 through the air inlet orifice 140, the gas pressure expands the flexible diffuser membrane 110 away from the diffuser body 120 and causes the membrane's perforations to open so that the gas discharges through them in the form of fine bubbles. When the gas pressure is relieved, the flexible diffuser membrane 110 collapses on the diffuser body 120 to close the perforations and prevent the liquid from entering the diffuser body 120 in the opposite direction. Generally, a flexible diffuser membrane 110 configured in this way produces bubbles smaller than five millimeters in diameter. The resultant large ratio of surface area to volume in these bubbles promotes efficient oxygen mass transfer between the bubbles and the surrounding wastewater. The fine bubbles also cause an upward movement in the wastewater tank, which helps to keep solid waste in suspension and to mix the contents of the tank.
A typical wastewater treatment tank may include 2,000 diffuser assemblies and their associated distribution conduits. Because of this large number, the ease in which the diffuser assemblies are mounted (i.e., mated) to the distribution conduits becomes a large factor in determining labor needs and, ultimately, installation costs. One such mounting method, for example, comprises the use of a clam-shell device or saddle that encircles the distribution conduit and provides a mounting point for the diffuser assembly. FIG. 2 shows such a saddle 200 on a distribution conduit 210 with a diffuser assembly 220. To install the diffuser assembly 220, a hole is first drilled in the distribution conduit 210 where the diffuser assembly 220 is to be placed, and then the saddle 200 is encircled about the distribution conduit 210 at this point and tightened thereto by hammering in a wedge 230. The diffuser assembly 220 is then attached to the top of the saddle 200. Water tight seals are ensured using two internal rubber o-rings (not shown). Another mounting method comprises the solvent welding of a plastic diffuser base to a plastic distribution conduit. The diffuser base and distribution conduit may, for instance, be formed of polyvinylchloride. A bead of resin around the solvent weld further ensures a watertight seal.
Nevertheless, while generally effective over the medium term when installed under ideal temperatures (e.g., above 30 degrees Fahrenheit) by a skilled foreman with a crew of semi-skilled workers, mounting diffuser assemblies in these manners is labor intensive and not conducive to using machines rather than humans to perform the mounting task. With respect to saddles, for example, there are too many parts for automation to be effective. Moreover, assembling plastic components together under sub freezing conditions often results in failure because plastic components contract and become brittle at low temperature. Accordingly, assembly is often delayed by the need to wait for a warm day or the need to soak the plastic components in warm water prior to assembly. With respect to solvent welding, the time required for a solvent weld and resin to cure renders automation of the attachment of the diffuser base to the distribution conduit prohibitively slow. Furthermore, the solvent weakens the joint, and, over the medium term, pipes have a tendency to crack and fail where solvent has been applied. Finally both examples of existing systems require a skilled foreman and a semi-skilled crew, which are costly and are not always available (e.g., in a National Parks or in a U.S. Island Territory).
Accordingly, it is desirable to obtain other means of mounting fine bubble diffuser assemblies to distribution conduits that utilize fewer parts and are less labor intensive than prior art means, but also provide the same level or a higher level of reliability in use. Ideally such other means will further lend themselves to the automation of the mounting process and the use of the same diffuser assembly for distribution conduits of varying shapes and dimensions.